This invention relates to an electrostatically assisted coating method and apparatus. More specifically, the invention relates to using electrostatic fields at the point of coating fluid contact with a moving web to achieve improved coating process uniformity.
Coating is the process of replacing the gas contacting a substrate, usually a solid surface such as a web, by one or more layers of fluid. A web is a relatively long flexible substrate or sheet of material, such as a plastic film, paper or synthetic paper, or a metal foil, or discrete parts or sheets. The web can be a continuous belt. A coating fluid is functionally useful when applied to the surface of a substrate. Examples of coating fluids are liquids for forming photographic emulsion layers, release layers, priming layers, base layers, protective layers, lubricant layers, magnetic layers, adhesive layers, decorative layers, and coloring layers.
After the deposition, a coating can remain a fluid such as in the application of lubricating oil to metal in metal coil processing or the application of chemical reactants to activate or chemically transform a substrate surface. Alternatively, the coating can be dried if it contains a volatile fluid to leave behind a solid coat such as a paint, or can be cured or in some other way solidified to a functional coating such as a release coating to which a pressure-sensitive adhesive will not aggressively stick. Methods of applying coatings are discussed in Cohen, E. D. and Gutoff, E. B., Modern Coating and Drying Technology, VCH Publishers, New York 1992 and Satas, D., Web Processing and Converting Technology and Equipment, Van Vorstrand Reinhold Publishing Co., New York 1984.
The object in a precision coating application is typically to uniformly apply a coating fluid onto a substrate. In a web coating process, a moving web passes a coating station where a layer or layers of coating fluid is deposited onto at least one surface of the web. Uniformity of coating fluid application onto the web is affected by many factors, including web speed, web surface characteristics, coating fluid viscosity, coating fluid surface tension, and thickness of coating fluid application onto the web.
Electrostatic coating applications have been used in the printing and photographic areas, where roll and slide coating dominate and lower viscosity conductive fluids are used. Although the electrostatic forces applied to the coating area can delay the onset of entrained air and result in the ability to run at higher web speeds, the electrostatic field that attracts the coating fluid to the web is fairly broad. One known method of applying the electrostatic fields employs precharging the web (applying charges to the web before the coating station). Another known method employs an energized support roll beneath the web at the coating station. Methods of precharging the web include corona wire charging and charged brushes. Methods of energizing a support roll include conductive elevated electrical potential rolls, nonconductive roll surfaces that are precharged, and powered semiconductive rolls. While these methods do deliver electrostatic charges to the coating area, they do not present a highly focused electrostatic field at the coater. For example, for curtain coating with a precharged web, the fluid is attracted to the web and the equilibrium position of the fluid/web contact line (wetting line) is determined by a balance of forces. The electrostatic field pulls the coating fluid to the web and pulls the coating fluid upweb. The motion of the web creates a force which tends to drag the wetting line downweb. Thus, when other process conditions remain constant, higher electrostatic forces or lower line speeds result in the wetting line being drawn upweb. Additionally, if some flow variation exists in the crossweb flow of the coating fluid, the lower flow areas are generally drawn further upweb, and the higher flow areas are generally drawn further downweb. These situations can result in decreased coating thickness uniformity. Also, process stability is less than desired because the wetting line is not stable but depends on a number of factors.
There are many patents that describe electrostatically-assisted coating. Some deal with the coating specifics, others with the charging specifics. The following are some representative patents. U.S. Pat. No. 3,052,131 discloses coating an aqueous dispersion using either roll charging or web precharging, U.S. Pat. No. 2,952,559 discloses slide coating emulsions with web precharging, and U.S. Pat. No. 3,206,323 discloses viscous fluid coating with web precharging.
U.S. Pat. No. 4,837,045 teaches using a low surface energy undercoating layer for gelatins with a DC voltage on the backup roller. A coating fluid that can be used with this method include a gelatin, magnetic, lubricant, or adhesive layer of either a water soluble or organic nature. The coating method can include slide, roller bead, spray, extrusion, or curtain coating.
EP 390774 B1 relates to high speed curtain coating of fluids at speeds of at least 250 cm/sec (492 fl/min), using a pre-applied electrostatic charge, and where the ratio of the magnitude of charge (volts) to speed (cm/sec) is at least 1:1.
U.S. Pat. No. 5,609,923 discloses a method of curtain coating a moving support where the maximum practical coating speed is increased. Charge may be applied before the coating point or at the coating point by a backing roller. This patent refers to techniques for generating electrostatic voltage as being well known, suggesting that it is referring to the listed examples of a roll beneath the coating point or previous patents where corona charging occurs before coating. This patent also discloses corona charging. The disclosed technique is to transfer the charge to the web with a corona, roll, or bristle brush before the coating point to set up the electrostatic field on the web before the coating is added.
FIGS. 1 and 2 show known techniques for electrostatically assisting coating applications. In FIG. 1, a web 20 moves longitudinally (in the direction of arrows 22) past a coating station 24. The web 20 has a first major side 26 and a second major side 28. At the coating station 24, a coating fluid applicator 30 laterally dispenses a stream of coating fluid 32 onto the first side 26 of the web 20. Accordingly, downstream from the coating station 24, the web 20 bears a coating 34 of the coating fluid 32.
In FIG. 1, an electrostatic coating assist for the coating process is provided by applying electrostatic charges to the first side 26 of the web 20 at a charge application station 36 spaced longitudinally upstream from the coating station 24 (the charges could alternatively be applied to the second side 28 of the web 20). At the charge application station 36, a laterally disposed corona discharge wire 38 applies positive (or negative) electrical charges 39 to the web 20. The wire 38 can be on either the first or second side of the web 20. The coating fluid 32 is grounded (such as by grounding the coating fluid applicator 30), and is electrostatically attracted to the charged web 20 at the coating station 24. A laterally disposed air dam 40 can be disposed adjacent and upstream of the coating station 24 to reduce web boundary layer air interference at the coating fluid-web interface 41. The corona wire could be aligned in free space along the web (as shown in FIG. 1) or alternatively, could be aligned adjacent the first side of the web while the web is in contact with a backing roll at the coating station.
FIG. 2 shows another known electrostatically assisted coating system. In this arrangement, a relatively large diameter backing roll 42 supports the second side 28 of the web 20 at the coating station 24. The backing roll 42 can be a charged dielectric roll, a powered semiconductive roll, or a conductive roll. The conductive and semiconductive rolls can be charged by a high voltage power supply. With a dielectric roll, the roll can be provided with electrical charges by suitable means, such as a corona charging assembly 43. Regardless of the type of backing roll 42 or its means of being charged, its outer cylindrical surface 44 is adapted to deliver the electrical charges 39 to the second side 28 of the web 20. As shown in FIG. 2, the electrical charges 39 from the backing roll 42 are positive charges, and the coating fluid 32 is grounded by grounding the coating fluid applicator 30. Accordingly, the coating fluid 32 is electrostatically attracted to charges residing at the interface between the web 20 and the outer cylindrical surface 44 of the roll 42. The air dam 40 reduces web boundary layer air interference at the coating fluid-web interface 41.
Known electrostatically assisted coating arrangements such as those shown in FIGS. 1 and 2 assist the coating process by delaying the onset of air entrainment and improving the wetting characteristics at the coating wetting line. However, they apply charges to the web at a location substantially upstream from the wetting line, and generate fairly broad electrostatic fields. They are largely ineffective in maintaining a straight wetting line when there are crossweb coating flow variations or cross-web electrostatic field variations. For instance, in a curtain coater, if a localized heavy coating fluid flow area occurs somewhere across the curtain, the wetting line in this heavier coating region can move downweb in response, depending on the material and process parameters. This can create an even heavier coating in this area due to stress and strain on the curtain, especially for fluids which exhibit elastic characteristics (more elastic fluids have high extensional viscosity in relation to shear). In addition, if the electrostatic field is not uniform (e.g., there is a corona web precharge non-uniformity), the lower voltage area on the web will allow the wetting line in that area to move downweb, thus increasing the coating weight in that area. These effects become increasingly dominant as fluid elasticities increase. Thus, crossweb fluid flow variations and crossweb electrostatic field variations cause non-uniformity in the wetting line and, as a result, the application of a non-uniform coating on the web.
None of the known apparatus or methods for electrostatically assisted coating discloses a technique for applying a focused electrical field to the web at the coating station from an electrical field applicator to improve the characteristic of the applied fluid coating and also to attain improved processing conditions. There is a need for an electrostatically assisted coating technique that applies a more focused electrical field to the web at the coating station.
The invention is a method of applying a fluid coating onto a substrate. The substrate has a first surface and a second surface. The method includes providing relative longitudinal movement between the substrate and a fluid coating station and forming a fluid wetting line by introducing, at an angle of from 0 degrees through 180 degrees, a stream of fluid onto the first side of the substrate along a laterally disposed fluid-web contact area at the coating station. An electrical force is created on the fluid from an electrical field originating from electrical charges which are on the second side of the substrate substantially at and downstream of the fluid wetting line.
The electrical force can be created by transferring the electrical charges through a fluid medium (e.g., air) and depositing the electrical charges onto the second surface of the substrate, or transferring electrical charges from a charge source and depositing the electrical charges onto the second surface of the substrate using physical contact between a portion of the charge source and the substrate, or both. When a fluid medium is used, the electrical charges can be transferred from a laterally extending corona discharge source closely spaced from the second surface of the substrate at the fluid coating station. The transfer of electrical charges upstream from the fluid wetting line can be further limited by providing an electrical barrier for shielding upweb portions of the web from the electrical charges. The substrate can be supported, adjacent the fluid coating station, on the second surface thereof.
In one embodiment, the electrical charges are formed as first charges at a location distant from the substrate, transferred to a laterally disposed charge application zone adjacent the second surface of the substrate at the fluid wetting line, and applied onto the second surface of the substrate at a location on the substrate that is substantially at and downstream of the fluid wetting line to create an electrical force on the fluid.
The stream of fluid can be formed with a coating fluid dispenser such as a curtain coater, a bead coater, an extrusion coater, carrier fluid coating methods, a slide coater, a knife coater, a jet coater, a notch bar, a roll coater or a fluid bearing coater. The stream of fluid can be tangentially introduced onto the first surface of the substrate.
The electrical charges can have a first polarity and the method can include applying second opposite polarity electrical charges to the fluid.
In another embodiment, the method of applying a fluid coating onto a substrate (where the substrate has a first surface on a first side thereof and a second surface on a second side thereof) includes providing relative longitudinal movement between the substrate and a fluid coating station. The method further includes forming a fluid wetting line by introducing, at a angle of 0 degrees through 180 degrees, a stream of coating fluid onto the first surface of the substrate along a laterally disposed fluid-web contact area at the coating station. The method further includes exposing effective electrostatic charges on the substrate to the fluid only at a location on the substrate that is substantially at and downstream of the fluid wetting line.
In this inventive method, the exposing step can further comprise depositing the electrical charges onto one of the first or second sides of the substrate at a location upweb from the fluid coating station. The exposing step can further include rendering the electrical charges ineffective as electrostatic charges relative to the fluid until the electrical charges are at least substantially at the fluid wetting line.
In one preferred embodiment, the exposing step of the inventive method further includes applying electrical charges to the substrate upweb from the fluid wetting line, and masking any effective electrostatic attractive forces between the electrical charges on the web and the fluid until the electrical charges are at least substantially at the fluid wetting line.
In a preferred embodiment, the electrical charges are applied to the first surface of the substrate and the masking step further comprises providing a grounded surface adjacent and spaced from the second surface of the substrate, with the grounded surface extending along the substrate from a trailing edge just upweb of the fluid wetting line to a leading edge spaced upweb further therefrom.
The invention is also an apparatus for applying a coating fluid onto a substrate which has a first surface on a first side thereof and a second surface on a second side thereof and is moved longitudinally relative to the apparatus. The apparatus includes means for dispensing a stream of coating fluid onto the first surface of the substrate to form a fluid wetting line along a laterally disposed fluid-web contact area and an electrical charge applicator extending laterally across the second side of the substrate. The electrical charge applicator is aligned generally opposite the fluid wetting line on the first surface of the substrate to charge the substrate at a location on the substrate that is substantially at and downstream of the fluid wetting line.
The electrical charge applicator can include a laterally extending charged wire, a sharp-edged member, a sharp-edged conductive sheet, a series of needles, a brush, or a jagged knife edge.
The electrical charge applicator can include an electrical charge source, for producing electrical charges as first electrical charges, distant from the second surface of the substrate, and a fluid medium. The fluid medium is disposed between the electrical charge source and the second surface of the substrate to transfer the first electrical charges from the electrical charge source to a laterally disposed charge application zone adjacent the second surface of the substrate at the fluid wetting line and to apply the first electrical charges onto the second surface of the substrate. The electrical charge applicator can be uniformly spaced from the second surface of the substrate.
An air bearing can extend laterally across the substrate adjacent the electrical charge applicator for supporting and aligning the second side of the substrate relative to the electrical charge applicator. An electrostatic field barrier can be disposed near the electrical charge applicator and the substrate to shield portions of the web upstream from the fluid wetting line from electrical charges from the electrical charge applicator.
Electrical charges from the electrical charge applicator can have a first polarity, and charges having a second, opposite polarity can be applied to the coating fluid.
The inventive method is also defined as a method of applying a fluid coating onto a substrate, where the substrate has a first side and a second side. The inventive method includes providing relative longitudinal movement between the substrate and a fluid coating station. A stream of fluid is introduced, at an angle of 0 degrees through 180 degrees, onto the first side of the substrate to form a fluid wetting line along a laterally disposed fluid-web contact area at the coating station. The invention further includes attracting the fluid to the first side of the substrate at a location on the substrate that is substantially at and downstream of the fluid wetting line by electrical forces from an effective electrical field originating at a location on the second side of the substrate.